fix: do not throw when there's no sidenav.

Fixes angular#269.

src/components/sidenav/sidenav.ts

@@ -19,11 +19,6 @@ import {Dir} from '../../core/rtl/dir';
 import {PromiseCompleter} from 'angular2/src/facade/promise';
 
 
-/**
- * Exception thrown when a MdSidenavLayout is missing both sidenavs.
- */
-export class MdMissingSidenavException extends BaseException {}
-
 /**
  * Exception thrown when two MdSidenav are matching the same side.
  */
@@ -286,9 +281,6 @@ export class MdSidenavLayout implements AfterContentInit {
    */
   private _validateDrawers() {
     this._start = this._end = null;
-    if (this._sidenavs.length === 0) {
-      throw new MdMissingSidenavException();
-    }
 
     // Ensure that we have at most one start and one end sidenav.
     this._sidenavs.forEach(sidenav => {